Several methods are known for determining from measuring values the silver concentration in fixing baths of film processing machines.
For controlling electrolysis cells, for example, the voltage potentials are measured via a cell anode and cathode, said potentials being more or less proportional to the silver concentration. Also known is the measuring of the conductivity of the fixing solution by means of conductivity sensors. In EP-O 418 757 A2, for example, it is proposed that the supply of replenisher solution for two fixing baths from two supply tanks be controlled by means of a single conductivity sensor.